The present invention concerns the neutralization of coordination catalyst residues, particularly residues of Ziegler-type catalysts useful in olefin polymerization processes.
It is well known that the residues of support based coordination catalysts, especially Ziegler-Natta catalysts, in stabilized polymers leads to a decrease in the thermooxidative stability of the polymers. This effect is associated with reactions occurring between the catalyst residues and additives in the polymer during high temperature processing and generally lead to increased discoloration of the polymer. It is difficult to characterize these reactive residues because their composition depends not only on the conditions under which the polymerization was carried out but also upon any subsequent processes in which the polymer has come into contact with oxygen and moisture.
It is important to note that there is an important distinction between additives which deactivate a coordination catalyst and additives which neutralize the catalyst residues. Deactivation of these catalysts is necessary to eliminate post polymerization reactions in process equipment and may be accomplished using any of the typical catalyst "poisons" known; i.e., O.sub.2, H.sub.2 O, alcohol, etc. However, in many cases, the deactivated catalyst residues are in themselves highly reactive at elevated temperatures in the presence of oxygen or moisture. This may lead to limited corrosion of metal processing equipment in a manner similar to that observed in a non-treated, non-deactivated polymer.
Neutralization of the catalyst residues typically also involves deactivation of the catalyst. In addition, however, these neutralizing compounds complex with the catalyst residues in a manner so as to render them inactive, or non-corrosive, during subsequent processing.
Transition metal-derived polyethylene catalysts have been deactivated and neutralized by contact with salts of carboxylic acids as disclosed in U.S. Pat. No. 4,105,609. While the metal salts disclosed in this patent are suitable for neutralizing Ziegler catalyst residues employed in polymerizing polymers prepared with Ziegler catalyst and preventing, to some degree, corrosion in metal processing equipment, they suffer from at least two serious disadvantages: (1) they are fairly insoluble in most commonly employed solvents, making addition to the polymer difficult at best, and (2) they tend to plate out on the surfaces of molding or casting equipment, causing an undesirable build-up of the metal salt on the equipment and necessitating production shutdowns to clean the equipment.
The problem of equipement plate-out or build-up, such as observed on mold plates of injection molding equipment or on the chill roll of a cast film unit, associated with the carboxylic acid metal salts mentioned above is reduced or eliminated by employing the "pseudo-polyester" neutralization agents of the present invention. Also, the initial color of the polymers containing Ziegler catalyst residues which have subsequently been neutralized according to the present invention are at least equivalent to examples using previous teachings in that they are more white and less yellow than samples containing no such neutralization agent. In addition, in many cases these "pseudo-polyesters" may function as a processing aid during extrusion of the neutralized material.